There has been known an electric power steering system for applying a proper assisting force to a steered member such as wheels from an electric motor in accordance with a torque in-putted from an input portion such as a steering wheel. The structure of the electric motor to be used in such electric power steering system has been proposed by us in Unexamined Published Japanese Patent Application No. 7-203888, for example.
FIG. 18 shows this electric motor of the electric power steering system of the prior art. As shown, an electric motor 201 is fixed in an assembling hole 204 which is formed in a wall 203 of a board case 202. In the case 202, a circuit board 205 is fitted to the wall 203. On this circuit board 205, a drive circuit comprising a plurality of FETs (Field Effect Transistors) 206 acting as switching elements for driving the electric motor 201, and a control circuit comprising a CPU 207 are installed.
An output shaft 210 extending from the electric motor 201 further extends through the assembling hole 204. It is supported on a cover 211 of the board case 202 via a bearing 212. And associated with a worm shaft 214 accommodated in a gear case 213. The worm shaft 214 is associated with a steered member, not shown, to transmit the assisting force from the electric motor 201 to the steered member.
Brushes 216 in sliding contact with a commutator 215 on the outer circumference of the output shaft 210 are held by brush holders 217, which are supported on an insulating spacer 218 mounted in the assembling hole 204. Pigtail wires (or lead wires) 219 for conducting the brushes 216 and the circuit board 205 are directly mounted on the circuit board 205.
The electric motor 201 of the prior art is thus constructed, but this structure has the following problems.
First of all, this electric motor 201 of the prior art cannot retain a sufficient heat releasing path from the plurality of FETs 206 composing the drive circuit and from the output shaft 210.
The plurality of FETs 206 composing the drive circuit on the circuit board 205 are usually arranged in the vicinity of the brushes 216, i.e., in the vicinity of the output shaft 210 from conveniences of the circuit construction. Since these FETs 206 generate heat the releasing path for this heat has to be retained. According to this prior art motor, however, the brushes 216 are located in the assembling hole 204 of the electric motor 201 and the FETs are also located in the vicinity of the hole 204, so the heat releasing path from the FETs 206 and the output shaft 210 is restricted to the path through the circuit board 205. Even if the board case 202 and the circuit board 205 are made of aluminum or the like having a high thermal conductivity, therefore, the heat release from the FETs 206 and the output shaft 210 may not be sufficient.
Secondly, this prior art motor has a problem that the temperature of the circuit board 205 becomes high when the pigtail wires 219 are fixed thereon. Specifically, the pigtail wires 219 extending from the brushes 216 are directly fixed on the circuit board 205, and this fixing operation is made by the hot-soldering or ultrasonic-welding work. As a result, the heat generated by this hot-soldering or ultrasonic-welding operation is hard to be released from the circuit board 205. In the worst case, the circuit pattern of the circuit board 205 may be separated by that heat.
Thirdly, this prior art motor is not convenient for carrying the dynamic characteristic tests of the electric motor 201 by itself. In this structure of the prior art motor, more specifically, the output shaft 210 of the electric motor 201 extends downward of the board case body 203 holding the circuit board 205 and it is supported on the cover 211 via the bearing 212. As a result, the dynamic characteristic tests of the electric motor 201 in the state detached from the case 202 have required a special jig for supporting the electric motor 201.
Fourthly, a torque limiter has to be provided for the electric motor 201. In this prior art motor, the torque limiter has to be arranged outside of the board case 202 which makes the structure of the entire system large-sized.
This invention has been conceived noting those problems and has an object to provide an electric motor for an electric power steering system, which has an excellent heat releasing property from the circuit board (especially, FETs composing the drive circuit) and the output shaft.
Another object of this invention is to provide an electric motor for an electric power steering system, which is not adversely affected by the high temperature of the circuit board at a step of electrically connecting the brushes and the circuit board of the electric motor.
Still another object of this invention is to provide an electric motor for an electric power steering system, in which the dynamic characteristic tests of the electric motor can be easily performed.
Still another object of this invention is to provide an electric motor for an electric power steering system, which can accommodate a torque limiter compactly in the electric motor thereby to reduce the size of the entire system.